In the early state of the art, fermentation processes were carried out on surfaces of solid media. However, surface fermentations are costly and difficult to operate. Thus, the liquid or submerged fermentation evolved. In submerged fermentation, particularly aerobic fermentation, air is an integral factor in the operation. The vessels used in submerged fermentation are called fermentors or bioreactors (the latter is preferably used when the vessel is designed for the culturing of tissue cells).
A fermentor is a vessel designed for the cultivation of microorganisms. The environment in the vessel is closely controlled to enable the proper expression of biochemical reactions for the production of the desired by-product. An important component for maintaining an aerobic environment is a system for gas dispersion and a means of transferring mass between the gas and the liquid phases. Fermentor design has not changed radically over the years, however, recently advances in biotechnology have spawned an array of compounds produced by microorganisms. User requirements are becoming increasingly sophisticated and complex, thus underscoring the need for improved culture vessels and systems.
To effect dispersion of air and media into the fermentation liquid and mixing therein, impellers are provided. The impellers used in fermentors are generally classified as flat-blade turbine (disc, vaned, and open), pitched-blade turbine or marine propeller. The marine propeller is essentially designed for mixing or suspending the solids in the medium, while the flat-blade turbine is for gas dispersion. Gas is efficiently dispersed when large gas-liquid interfacial surface areas per unit volume of the gas-liquid system are created.
When gas is sparged into the bottom of a vessel, the gas travels to the liquid surface as gas bubbles. These bubbles are relatively large and poorly distributed, particularly in the lower portion of the vessel. Since some interfacial areas are created by the gas as it rises to the surface, some mass transfer is possible. However, the contents of the vessel would likely be non-uniformly mixed because the only agitation is provided by the gas sparge.
When a gas bubble rises without being dispersed, the condition is known as flooding. To avoid flooding, the fermentor is equipped with a turbine agitator; the degree of dispersion is dependent on the speed of agitation. As the rotational speed is increased, bubbles are driven radially and eventually driven forcefully to the vessel wall and below the impeller. Thus, the horsepower required to prevent flooding is equivalent to that which is necessary to support minimum agitation. Turbine agitators for gas dispersion become larger and more power intensive with increasing liquid volume.
There are two basic ways in which an impeller can be used in the dual role of aeration and agitation. The first involves the use of a coaxial impeller in a tank without baffles; if the impeller turns fast enough, then a vortex is drawn down from the liquid surface to meet the impeller. This type of fermentor system is known as "vortex" aeration. The second system which is most commonly employed uses a series of vertical baffles (parallel to and spaced apart from the impeller) to prevent liquid swirl and thereby enables the impeller to create liquid turbulence.
The ultimate objective of gas dispersion or aeration is to maximize the oxygen absorption rate (OAR), i.e., for the oxygen to be transferred from the gas phase to the liquid, and from the liquid to the microbial cells. Because oxygen has a very low solubility (5.46 ml oxygen/liter) in aqueous culture solutions, it must be continuously supplied and dispersed. The impeller "breaks up" the air into small bubbles that are dispersed throughout the liquid. The agitation by the impeller can also aid in removing carbon dioxide and other metabolic gases from the medium. For mammalian and plant cell cultures, however, conventional agitation can cause contact damage (traumatic damage) to the cells. These tissue cells require low shear, and thereby gentle agitation for optimum growth. In this regard, the standard fermentor is not ideally designed for the cultivation of tissue cells.
A low shear condition is provided by the bubble column design. In a bubble column, air is simply bubbled or sparged into the liquid container. Thus, the bubble column has been modified for use as a fermentor/bioreactor. The resultant modifications include the tower and air-lift fermentors. In a tower fermentor, several perforated plates are placed in the column. Instead of perforated plates, motionless gas diffusors (stainless steel sheets bent into wave shape and put together with layers parallel to each other) can be used. Another type of tower fermentor involves addition of a recycling loop and the use of baffles.
The air-lift fermentor involves a draft "tube" submerged in the liquid medium inside the column. The air-lift fermentor applies air for circulating the medium: air and liquid flow concurrently upwards in one part of the column, resulting in a corresponding downward draft in another part. Another type of air-lift fermentor employs "pulsating aeration" in which air passes backward and forward through a single air filter. Recent variation in the air-lift fermentor involves a change in the shape of the vessel, particularly a tapered column instead of a cylindrical vessel. Despite these various improvements and despite the need for even more improved systems and technology, the same have not been available.
During the growth of a microbial culture in the fermentation vessel, particularly in an impeller agitated vessel, foaming is a recurring problem. Uncontrolled foaming may result in considerable loss of the fermentation/cultivation medium due to foam-over. Foaming may also lead to wetting of exhaust filters and consequent contamination of the batch or plugging of the filters. The factors that contribute to the formation of stable foams are relatively high surface tension and viscosity, and presence of finely divided solids.
Foaming may be controlled by mechanical means or by the addition of chemical antifoam agents. An example of a conventional mechanical foam breaking device is an array of specially shaped discs fitted on to the fermentor shaft. The foam is sucked into the discs and is destroyed by virtue of a weir arrangement. Other conventional means include a draught tube which acts as a weir for the liquid and foam to spill over; passing effluent air and foam through a nozzle at accelerated velocities, the resultant liquid being deflected down towards a pump for recycle into the fermentor (the system being limited however to solid-free cultures); and destruction of foam by ultrasonic waves, usually effective only at high air velocity.
Because of the limitations of existing mechanical foam breakers, chemical antifoam agents are more commonly used. The chemical agents act by competitively replacing the surface active agents responsible for causing the foam and themselves being unable to produce stable foams, e.g., silicones, polyglycol, and liquid paraffins. Although chemical antifoam agents are generally effective, certain microorganisms may be inhibited by such agents. Also, residual antifoam agents may interfere in the downstream processing of the fermentation broth.